<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            border: 1px solid #000;
            display: inline-block;
        }
    </style>
</head>
<body>
    <div>
        <button>red</button>
        <button>yellow</button>
        <button>blue</button>
        <button>green</button>
    </div>
    <br>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>

    <script>
        let aBtn = document.getElementsByTagName("button");
        let aBox = document.getElementsByClassName("box");
        let color = null;
        for(let i = 0;i < aBtn.length;i++){
            aBtn[i].onclick = function(){
                color = this.innerHTML;
            }
        }

        for(let i = 0;i < aBox.length;i++){
            aBox[i].onclick = function(){
                if(color != null){
                    aBox[i].style.background = color;
                }
            }
        }
    </script>
</body>
</html>